This invention is in the general field of electrochemical conversion, using, for example, electrochemical cells.
Pressing requirements for clean transportation, load leveling of electric utilities, as well as many other electrochemical applications have promoted significant research for new electrochemical cells. Energy density, cost, cycle life, recharge efficacy, safety, environmental effects, and serviceability are among the factors to be considered in producing a battery suitable for practical use in many applications.
The ability to convert chemical to electrical energy and back again has been well known for almost two centuries. However, certain applications, such as electric vehicles, have requirements for energy density, low cost, and long cycle life which are difficult to meet when constructing commercially practical cells that are operable and safe. For example, a high theoretical energy density (see the discussion of this term below) may in some cases be associated with increased weight of the components, thereby undercutting the theoretical advantages.
I have discovered that the use of boron redox species can provide an electrochemical cell with a favorable balance of characteristics, such as available energy, energy density, capital and operating cost, recharge efficiency, safety, environmental impact, serviceability and longevity.
Accordingly one aspect of the invention generally features an electrochemical storage medium comprising a carrier mixed with a reduced boron-containing compound (preferably borohydride), the reduced compound being oxidizable to an oxidized boron-containing compound (preferably borate or polyborates; in non-aqueous systems using a halogen-containing reducing agent, borontrichloride may be produced) concurrent with the generation of an electric current when the storage medium is in electrical contact with an electrode that carries current generated during that oxidation. The carrier may be an aqueous or a non-aqueous solution, e.g., a liquid that dissolves the reduced compound and contacts the electrode so that the reduced boron-containing compound can provide electrons directly to the electrode, rather than indirectly through a stable intermediate such as hydrogen. Preferred non-aqueous liquids include anhydrous ammonia; dimethylformamide; dimethylsulfoxide, amines; non-amine organic bases; alcohols; alkene carbonates; and glycols; specific liquids include tripropylamine; pyridine; quinoline; triethanolamine; monoethanolamine; ethylene glycol; propylene glycol; methanol; ethanol; ethylene carbonate; and propylene carbonate. The non-aqueous solution may include a solubilizer or a conductivity enhancer, such as EDTA, crown ethers, cryptates, and quaternary ammonium salts.
A particularly interesting embodiment of this aspect of the invention includes a redox cycling pair that acts semi-catalytically to aid in the current generation. The redox cycling pair is chosen so that oxidized member of the redox cycling pair is reducible by borohydride to yield the reduced member of the redox cycling pair and borate, and the reduced member of the redox pair participates in a redox cycle which regenerates the oxidized member of the redox cycling pair while donating electrons. The storage medium is in electrical contact with an electrode for receiving those electrons in a current. Specific redox cycling pairs include: those having a metal hydride alloy as the reduced member; those having palladium or a palladium alloy (e.g., palladium/silver) as the reduced member; those having a metal as the reduced member; those having a metal hydride as the reduced member, as defined below in greater detail. Specific pairs are: gallium/gallate, sulfite/thiosulfate, Sn(OH)6/HSnO2, Mn/Mn2+, PO3xe2x88x922/HPO3xe2x88x922, Cr/CrO2, Te/Texe2x88x922, and Se/Sexe2x88x922. Desirably, the hydrogen pressure of the reduced member of the redox pair is less than 760 mm, preferably less than 38 mm. The above described redox cycling pair can be used with the battery and other aspects of the invention described below.
In another aspect of the invention, the storage medium is positioned to be the anode of a battery, which includes an anode and a cathode in electrical communication. The reduced compound is oxidizable to an oxidized boron-containing compound concurrent with the discharge of the battery, e.g., when the reduced compound contacts the electrode and delivers electrons to it. Available air may be the oxidizing agent, or the battery may include an oxidizing agent, such as: O2; compounds comprising oxygen and a halogen; and X2, where X is a halogen. Preferred agents are: perchlorate (ClO4xe2x88x92), chlorate (ClO3xe2x88x92), chlorite (ClO2xe2x88x92), hypochlorite (OClxe2x88x92), chlorine (Cl2), bromine (Br2), bromate (BrO3xe2x88x92) iodate (IO3xe2x88x92) or other comparable halogen/oxygen compounds. Other preferred agents are those which contain elements that may easily change between two or more oxidation states, in general, starting in the higher state. These compounds may or may not be soluble in the carrier medium, and they may be used as a solution, slurry, paste, gel or any other desired form. Preferred agents include: a) [Mn(VII)O4]xe2x88x92 (e.g., sodium permanganate); b) [Fe(VI)O4]xe2x88x922 (e.g., sodium ferrate); c) Ce (IV)OH(NO3)3 (basic cerium nitrate); d) [Ce(IV)(NO3)6]xe2x88x922 (e.g., as ammonium cerium nitrate); e) [Fe(III)(CN)4]xe2x88x923 (ferricyanide); f) [Cr(VI)O4]xe2x88x922 (chromate); g) [Sn(IV)O3]xe2x88x922 (stannate); h) [Bi(V)O3]xe2x88x92 (bismuthate); i) Mn(IV)O2; j) Ag(I)2O; k) Ag(II)O; l) Ce(IV)O2; m) Pb(IV)O2; n) Ni(III)O(OH); o) Ni(IV)O2; p) Co(III)O(OH); q) [N(V)O3]xe2x88x92 (e.g., ammonium nitrate, sodium nitrate, lithium nitrate, calcium nitrate); r) [NO2]xe2x88x92 (e.g., sodium nitrite); s) [S2O8]xe2x88x922 (e.g., ammonium or sodium peroxydisulfate); t) compounds containing Cu(III), Tl(III), Hg (II), Se (VI), or Te(VI); or u) R(NO2)n where R is an alkyl, aryl, or arylakyl organic group and n=1-6 (e.g., mono- or poly- or pernitro organic compounds). Note that valences are supplied simply to aid in understanding the nature of the oxidizing agents, but not necessarily as a claim limitation. Still other oxidizing agents are trinitrobenzoic acid, hexanitrobenzene, or trinitrobenzene.
The anolyte and catholyte of the battery may be separated by a permiselective membrane, such as an anionic membrane, a cationic membrane, or a bipolar membrane. The cathode may be an air breathing cathode, e.g., with a catholyte which can be oxidized by air (e.g., in basic solution) to produce an agent that then oxidizes borohydride to borate with the generation of electrical current, preferably in a cycle that includes regenerating the catholyte after it has generated electricity by oxidizing the borohydride, thus allowing its reuse. For example, the catholyte can contain iodate (IO3); ferricyanide and ferrocyanide; chromate and Cr+3; manganese at valence +2 and +3; tin at valence +2 and +4; Cobalt at valence +2 and +3; a catalyst to aid the reoxidation of the oxidation agent to the higher oxidation state by air. The battery may include a chamber separate from the cathode compartment in which reoxidation of the catholyte takes place. The battery may include two units, one that is the direct air breather, and another unit which comprises a catholyte which can be oxidized by air and can then oxidize borohydride to borate with the generation of electrical current using air indirectly. The battery may also include a bipolar electrode. It may have external storage tanks for storage of the anolyte, the catholyte, or both the anolyte and the catholyte. The cell to generate electricity by oxidation of borohydride may be physically separated from the cell to generate the borohydride from the borate, or it may be the same cell as that used to generate borohydride from borates. A controller may be connected to at least one source of a reactant, to determine the transport of the reactant to the anode or the cathode, and a monitor which determines a battery characteristic and produces a signal to the controller in response to monitored values of the characteristic. For example, at least one probe responds to a characteristic selected from ORP, conductivity, voltage, current and power output, ion concentration, pH, index of refraction, calorimetric, COD, turbidity, density to generate an input signal, the input signal being transmitted to an electronic processor, the processor being connected to the controller which controls flow into a battery compartment. The battery electrode may include a conductive substrate such as stainless steel which is coated. Electrodes that are particularly useful (e.g., to avoid generating hydrogen) include a) an alloy of bismuth, thallium, cadmium, tin, gallium, lead or indium; b) mercury or mercury amalgamated with other metals or mercury coated on a conductive substrate; c) tellurium or tellurides. The electrode may include additional materials to improve corrosion resistance or other properties of the electrode. The may be a bipolar electrode having two sides, one of the sides being coated with said substance, and a second side being coated with a material of low oxygen overvoltage such as gold, or iridium oxide; alternatively the second side is a standard air breathing electrode. The bipolar electrode comprises a sheet of conductive material, such as stainless steel or gold plated copper, or another appropriate metal. The electrode may have a smooth or high surface area of foam metal or tubes, cylinder, fibers, or another geometric shape, powder, coated or uncoated catalyzed or uncatalyzed.
Batteries as described above may be configured as a sealed unit of physical size and shape and correct voltage range to meet form fit and function specifications of a standard battery for a consumer electronic or electrical device. e.g, a button for a hearing aid; AAA; AA; A; B; C; D; 9 volt; a computer battery; a cellular phone battery. Also, the battery may be characterized by voltage and current production suitable for ignition and starter motor operation in a vehicle powered by an internal combustion engine, or it may be suitable for installation on a vehicle that uses electricity either partially or entirely to propel the vehicle. The battery may also be suitable for storage of electricity for applications such as electric utility load leveling and other means of storage of electricity. This aspect of the invention also features generating a current over time by connecting the battery to a load, in which case the current is generated by oxidation of the reduced boron-containing compound. The battery may be recharged by applying an electrical potential to the anode to regenerate borohydride anion from borate anion. Alternatively, discharged anolyte solution may be replaced with anolyte comprising borohydride anion suitable for oxidation to borate anion.
Another aspect of the invention features synthesizing a borane ion (borohydride) by electrical reduction of borate ion, e.g., as a method of recharging a battery. The synthesis may be monitored using of a probe which generates an electrical signal representative of a characteristic selected from ORP, conductivity, voltage, current and power input, ion concentration, pH, index of refraction, calorimetric, COD, turbidity, density, said signal being transmitted to an electronic processor, the processor being connected to the controller which controls flow into a battery compartment which is connected to a regulate flow into each compartment, e.g., via pumps, valves and other appropriate conveyances. Typically, the borate and borohydride ion are in an aqueous carrier. The cathode of the cell can be an electrode of high hydrogen overpotential, the reduction of borate ions being accomplished by applying a potential to an electrode which may be comprised of an alloy of bismuth, thallium, cadmium, tin, lead, gallium and indium. Alternatively, the electrode for application of a potential is comprised of mercury or mercury amalgamated with other metals or mercury coated on a conductive substrate. The electrode may also contain tellurium or tellurides. Such electrodes inhibit the release of hydrogen gas while current is passed through. Electrode additives to improve characteristics such as corrosion resistance may be added. Other highly reduced species (in addition to borohydride) may be recovered, e.g., species comprising metals or compounds from aqueous and/or non aqueous systems. As before, in this method, a bipolar electrode may be used; also, a permiselective membrane maybe used, e.g., an anionic membrane, a cationic membrane, or a bipolar membrane. Oxygen may be released from an anode while producing borohydride in a catholyte. An oxidized species may be produced as a product in an anode chamber. Non-borohydride boranes may be produced by adding partial reduction adducts or other adducts to the catholyte, such as cyanide ion, amide ion, halide ions, and pseudohalides.
In yet another aspect, the invention generally features transporting a borohydride anion from a generation point to a consumption point, by applying an electrical potential to a solution of oxidized borohydride at the generation point to produce borohydride in a first cell and transporting the borohydride solution to the consumption point where the borohydride is provided for oxidization in a second cell. Also, spent solution comprising oxidized borohydride may be transported from the consumption point to the generation site and applying the electrical potential to the spent solution at the generation point to produce borohydride in the first cell. The resulting borohydride may be used as described above. Alternatively, it may be combined with water to generate hydrogen by reduction of water, e.g., catalyzed by the presence of transition metal compounds such as a cobalt (II) compound (e.g., cobalt(II)hydroxide). The hydrogen may be collected and transported to a hydrogen consumption point, e.g., an industrial hydrogen user. The oxidized borohydride solution may be transported back to the generation point to be reused for generation of borohydride.
This system may be used with cells that are configured to be suitable for installation on a vehicle that uses electricity either partially or entirely to propel the vehicle, or they may be used for storage of electricity for applications such as electric utility load leveling and other means of storage of electricity. In short, this aspect features a system of transporting borohydride as a method of transporting energy to a given location, e.g., a system of transporting and distributing borohydride such that vehicles that operate with borohydride may fill up with fresh borohydride and discharge the borates. The borate solution is converted to borohydride solution with a cell for synthesizing borohydride, e.g., for recharging a battery, by electrical reduction of borate ion.
The system benefits from the very high energy of electrode couples based on the use of borohydrides at the anode. Additionally, the system is versatile because the reactants can be used in a wide variety of chemical environments, for both secondary and primary cells. xe2x80x9cSecondaryxe2x80x9d refers to the ability to recharge the cell and xe2x80x9cprimaryxe2x80x9d refers to a system where the original reactants are used only once and not regenerated by a charging reaction. Another feature that provides versatility is the solubility of reactants in water or other solvents, enabling configurations in which the reactants are flowed through a cell or kept stationary in a paste or gel or solution.
The reactants may be varied to include a wide variety of oxidizing agents for the cathode of the battery, including oxygen which may be taken in from ambient air. This flexibility allows many configurations from low power-long life to high power-long life as needed. The ability to store liquid fuel external to the cell is key to providing a long range system where electrode area is independent from the total energy stored, for example, in applications such as utility load leveling and automotive transportation. The spent liquid can be rapidly exchanged for fresh liquid instead of actually having to recharge the system using electricity.
The term xe2x80x9cElectroconversion Cellxe2x80x9d includes operation either to convert electricity to chemicals or chemicals to electricity and operation as either a primary cell or a secondary cell or a fuel cell or a synthesis cell or any combination of the above.
Cell refers to any electrochemical system whether producing or consuming electricity and/or producing and consuming chemicals. Battery refers to any combination of cells used to produce electricity. Primary cell refers to a cell designed to deliver electricity but not to be recharged. Secondary refers to a cell that can both deliver electricity and be recharged. Fuel cell refers to a cell that generates electricity by consuming xe2x80x9cfuelsxe2x80x9d such as hydrogen, hydrazine or methanol with oxygen and generally will produce electricity as long as fuels are supplied but does not reverse its function to recharge or produce the materials that it has consumed. As noted, Electroconversion Cells refers to cells defined below having the ability to function in one or more of several modes.
In the following equations, references to a voltage (E0 or Exc2xd) imply standard conditions and are provided for calculations only. Cells that operate at conditions (including concentrations, temperatures and pressures) other than standard will exhibit different voltages. Nothing about the voltage listed should be considered in any way as limiting the rich variety of cells and battery combinations offered by the Electroconversion Cell, nor does the listing imply past or future actual attainment of the listed voltage. E0 refers to the voltage of a complete reaction and E1/2 or E1/2 refers to a half reaction which occurs in one part of a cell while another half reaction must be occurring in the cell as well.
xe2x80x9cElectrodexe2x80x9d is a very broad term and refers to a conductive material that conducts electrons in or out of a cell. Anodes and cathodes are both electrodes. Bipolar electrodes (one side is the anode the opposite side is the cathode to an adjacent cell) are electrodes. Electrodes can be solid or liquid. Air breathing electrodes must interact with a gas as well. In recent years a very wide variety of electrodes has become available. Electrodes may be as simple as a sheet of steel or may be a xe2x80x9cfoam metalxe2x80x9d or colloidal, or a powder. Electrodes can be almost any shape including sheets (plates), tubes, cylinder, cubes, spheres or almost any shape that can be designed for a given purpose. They may have many characteristics, porous, non-porous, flow through, scavenger and on and on. While any number of these electrode types are useable in the Electroconversion Cells to obtain a desired result, none is specifically required to utilize the electrochemistry as revealed in this invention. Therefore, whenever electrodes are mentioned, any configuration of electrodes used in a real battery or cell that utilizes the chemistry as disclosed herein is considered as an embodiment of this invention.
When a cell is operating to discharge and thereby produce electricity the electrode that interacts with the xe2x80x9cfuelxe2x80x9d that gets oxidized is called the anode. The liquid in this chamber is called the anolyte. The other half of cell has an electrode called a cathode and the solution in this chamber is called the catholyte. In a cell with no barrier to maintain different chemistries (such as the common lead-acid battery) the anolyte and catholyte are one in the same (in the lead-acid battery it is sulfuric acid) and is often simply called the electrolyte. In any event electrolyte as used herein will refer to any conducting liquid or suspension in any function.
When a battery is being recharged, electricity is being put into the cells. Under these conditions the components functions reverse and the anode is now the cathode and visa versa.
An alternative nomenclature is to call the anode of the discharging cell the negative electrode. Indeed electrons flow out of this side of the battery and travel through the load to the positive side of the battery. During recharge electrons flow into the negative electrode and this designation does not change based on whether the battery is charging or discharging.
Energy is used in units of watt-hours (wh) or kilowatt hours (kwh) or kilojoules (kJ). Power is used in units of watts (w) or kilowatts (kw). Energy density refers to an amount of energy available from a certain volume of cell or material, expressed, for example as watt-hours per liter (wh/l) or in kilowatts per liter (kwh/l). Specific energy refers to energy available from a certain mass of cell or material, expressed as watt-hours per kilogram. (wh/kg) or kilowatts per kilograom (kwh/kg). Energy density and specific energy are related to each by the density of the material or system. Specific power refers to the amount of power available per unit weight usually in watts/kg. A sparger is a device that facilitates gas/liquid contact, for example, a device with fine pores that breaks up a gas flow into very fine bubbles to achieve a very high surface area interface with a liquid.
The inventor uses a unit of his own definition: the Volt-Faraday (xe2x80x9cV-Fxe2x80x9d). This V-F unit is found by multiplying the voltage of a reaction by the number of electrons participating in the reaction. It is a quick way to determine available energies for batteries. Thus, 37.31 V-F equals one kilowatt-hour (1000 watt-hours); one V-F equals 26.8 watt-hours.
In many examples the inventor calculates the theoretical energy density or theoretical specific energy. These are for comparison purposes and should not be construed as being achievable in any real battery system. Nor should any such number be considered as a requirement for any embodiment of the Electroconversion Cell. Theoretical calculations do not take into account any containers, electrodes, pumps or any auxiliary gear that may be required to make a real cell. Such calculations are useful to predict at the outset whether a particular chemistry has even a theoretical chance of achieving a certain goal.
Other embodiments will be apparent to those skilled in the art from the following description of the preferred embodiments and from the claims.